Automatic azimuth recovery system for cannons

ABSTRACT

The rear end of a mortar tube is supported by a base plate with the front end supported in an elevated position by a tripod. The tripod comprises a cylinder with a cooperating fluid actuated piston rod which carries a yoke that is pivotally connected to a sleeve slidingly disposed on the tube to vary the elevation angle thereof responsive to axial movement of the piston rod. The yoke is slidingly mounted to the piston rod for lateral displacement responsive to fluid pressure applied thereto. Rearward recoil displacement of the base plate relative to the tripod rotates the trunnions that pivotally mount the sleeve to the yoke. This angular movement of the trunnions is sensed by a fluidic system whereby the piston rod is actuated linearly to move the sleeve along the tube until the pre-adjusted elevation angle thereof is recovered. Any lateral movement of the base during the rearward recoil displacement rotates the piston rod about its central axis. A fluidic sensor on the cylinder senses the angular movement of the piston rod and through a fluidic system actuates the yoke laterally to where the pre-determined azimuth adjustment is quickly recovered simultaneously with the recovery of the preselected elevation angle.

United States Patent [191 Ziegler Jan. 1,1974

[ AUTOMATIC AZIMUTH RECOVERY SYSTEM FOR CANNONS William H. Ziegler, Waterford, NY.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

[22] Filed: June 27, 1972 [21] Appl. No.: 266,802

[75] Inventor:

Primary Examiner-Stephen C. Bentley Atlorney-Harry M. Saragovitz et a1.

[57] ABSTRACT The rear end of a mortar tube is supported by a base plate with the front end supported in an elevated position by a tripod. The tripod comprises a cylinder with a cooperating fluid actuated piston rod which carries a yoke that is pivotally connected to a sleeve slidingly disposed on the tube to vary the elevation angle thereof responsive to axial movement of the piston rod. The yoke is slidingly mounted to the piston rod for lateral displacement responsive to fluid pressure applied thereto. Rearward recoil displacement of the base plate relative to the tripod rotates the trunnions that pivotally mount the sleeve to the yoke. This angular movement of the trunnions is sensed by a fluidic system whereby the piston rod is actuated linearly to move the sleeve along the tube until the pre-adjusted elevation angle thereof is recovered. Any lateral movement of the base during the rearward recoil displacement rotates the piston rod about its central axis. A fluidic sensor on the cylinder senses the angular movement of the piston rod and through a fluidic system actuates the yoke laterally to where the predetermined azimuth adjustment is quickly recovered simultaneously with the recovery of the pre-selected elevation angle.

9 Claims, 13 Drawing Figures PATENTEDJAH H974 I 3782.243

SHEEI 10? 5 AMPLIFIER RE FLUID SUPPLY SENSOR DEVICE H CH PRESSURE FLUID SUPPLY (6O 1 LOW PRESSURE :3

FLUID SUPPLY .726

L AMPLIFIER I i a HIGH SENSOR SENbOR PRESSURE V I DEVICE FLUID DEvIcE SUPPLY [0O 5 28 SHEET 2 0F 5 PATENTEDJAH 1 m4 SHEET 30? 5 AUTOMATIC AZIMUTI-I RECOVERY SYSTEM FOR CANNONS The invention described herein may be manufactured, used and licensed by or for the Government without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION This invention relates to cannons and pertains more particularly to a control device for automatically recovering the pre-determined azimuth adjustment of a cannon, when displaced therefrom, simultaneously with the recovery of the angle of elevation.

Oftentimes cannons, because of the exigencies of war, have to be mounted on unstable terrain which permits the cannon to be displaced sufficiently, responsive to recoil forces, to move the cannon off target. This is particularly true of infantry mortars which are packed to areas, often of rough terrain, and then quickly set up, sighted on the target and fired. In such instances, a number of projectiles have to be fired before the mortar base is sufficently entrenched to withstand further recoil forces without appreciable displacement. Consequently, mortars have to be resighted after each of the initial rounds and, as the only known practical means of resighting a mortar is by manual manipulation, the resighting takes considerable time thereby reducing the firepower of the mortar.

In the copending patent application by Ronald J. LaSpisa and Gary Woods entitled Automatic Elevation Recovery System for Cannons, Ser. No. 242,004; filed Apr. 7, 1971, a system is provided for cannons whereby the pre-selected elevation angle is automatically and quickly recovered after recoil displacement of the supporting base plate. That system, however, is only responsive to the rearward component of the base plate displacement, which affects the elevation angle, and is not responsive to the lateral component of such displacement which affects the azimuth alighment of the cannon with the target.

SUMMARY OF THE INVENTION It is one object of this invention to provide for cannons a control system that automatically and quickly reestablishes complete alignment with the target after displacement therefrom.

It is a further object of this invention to provide in combination with an existing system for recovering a preselected elevation angle a system having cooperation therewith for automatically, quickly and simultaneously recovering the pre-determined azimuth setting.

It is a still further object of this invention to provide such an azimuth recovery system which is fluid actunions that pivotally mount the sleeve on the tripod for transmits a signal responsive to the angular movement to a fluidic amplifier whereby fluid of unbalanced pressure is transmitted to opposite ends of a cylinder for relative movement of a piston therein. The cylinder, piston and connecting piston rod form one leg of the tripod whereby the displacement of the piston is transmitted to the front end of the mortar tube to change the elevation thereof until it returns to the pre-selected elevation angle as sensed by the sensor.

When the base plate is also displaced laterally it rotates about the connecting rod axis. This causes the connecting rod to rotate relative to the cylinder. A sensor mounted on the cylinder in cooperation with the connecting rod senses the angular movement thereof and transmits a signal responsive thereto through a fluidic amplifier to a cylinder-piston assembly disposed between the tripod and sleeve. Actuation of the piston displaces the front end of the tube laterally to where it is returned to the pre-determined azimuth setting thereby balancing the signal generated by the sensor and, consequently, the fluid pressure on opposite sides of the piston.

The specific nature of the invention as well as other aspects and advantages thereof will clearly appear from the following description of a preferred embodiment which is illustrated in the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of the azimuth recovery system of the present invention in an automatic sight recovery system for cannons;

FIG. 2 is a schematic view of the cooperating elevation recovery system with those elements which are common to those in FIG. 1 having the same reference numbers;

FIG. 3 is a side view of a mortar showing the connecting conduits for the elevation recovery system;

FIG. 4 is a front view of a mortar showing the connecting conduits for the azimuth recovery system and the tube properly sighted on a target;

FIG. 4a is a diagrammatic view showing the relationship of the bore axis and the sight line when the mortar is properly sighted on a target, as shown in FIG. 4;

FIG. 5 is a view similar to FIG. 4, but with the conduits removed and the base plate displaced sideways thereby misaligning the tube respective to the target;

FIG. 5a is a view similar to FIG. 4a but shows the relationship of the bore axis with the sight line when the mortar tube is displaced as shown in FIG. 5;

FIG. 6 is a view similar to those of FIGS. 4 and 5 but shows the azimuth component of the target sight alignment recovered by the azimuth recovery system;

FIG. 6a is a view similar to those of FIGS. 4a and 5a but shows the relationship of the sight line and bore axis when the sight alignment is recovered as shown in FIG. 6;

FIG. 7 is a fragmentary front elevational view partially cross-sectioned to show a part of the sight recovery system and those members of the tripod in which the system elements are housed;

FIG. 8 is a fragmentary side elevational view of that part of the tripod which includes the sight recovery system;

FIG. 9 is a section taken along line 99 of FIG. 8; and

FIG. 10 is an enlarged section taken along line 10-10 of FIG. 8.

DESCRIPTION OF A PREFERRED EMBODIMENT Shown in the drawings is a mortar 12 comprising a firing tube 14, a base plate 16 pivotally mounted to the rear end of the tube for support thereof, a tripod 18 that slidingly supports the front end of the tube, and a sight recovery system 28 for automatically realigning the tube with the pre-determined sight line when displaced therefrom by relative displacement of the base plate and tripod. Base plate 16 is provided with a plurality of spikes 22 which project downwardly from the underside thereof so as to be entrenched into the supporting terrain responsive to recoil forces produced in tube 14 when a projectile is fired therefrom.

Tripod 18 comprises a cylinder 24 with a cooperating piston 26 slidingly disposed therein. A piston rod 28 extends from piston 26 and, together with cylinder 24, forms one leg of tripod 18 which is coincident with a vertical plane defined by the longitudinal axis of tube 14. The other two legs 29 of tripod 18 are attached to the upper end of cylinder 24 so as to spread oppositely and outwardly therefrom. A spiked plate 30 extends from the bottom end of cylinder 24 and similar spiked plates extend from the bottom ends of legs 29 to secure tripod 18 against displacement when the spikes are driven into the supporting terrain.

Fixed to the extending end of piston rod 28 is a car rier 32 having a flange 34 of T-configuration which is slidingly engaged by a T-slot 36 in yoke 38 thereby providing for sliding displacement thereof relative to the piston rod and normal to the longitudinal axis thereof. Yoke 38 comprises a laterally disposed base 40 and a pair of spaced arms 42 which extend from opposite ends thereof and such arms receive therebetween a sleeve 44 having a bore 46 that slidingly receives tube 14 allowing for both linear and angular displacement relative thereto. Sleeve 44 is mounted to arms 42 for pivotal rotation about a diametrical axis by means of a pair of trunnions 48 that extend from the sleeve and are received by cooperating holes 50 in the arms. Thus, axial displacement of piston rod 28 by piston moves sleeve 44 upwardly along the inclined tube 14 to vary the elevational angle thereof. The lateral displacement of yoke 38 relative to carrier 32 pivots tube 14 at base plate 16 to change the azimuth angle of the tube.

Sight recovery system 20 comprises an elevation recovery system 52 having an elevation sensor 54 located in one of the arms 42, and includes a rotor 56 which is angularly displaceable by the related trunnion 48 for angular displacement thereby as fully described in the hereinbefore referenced patent application by LaSpisa et al. As is more fully disclosed in such patent application, the angular movement of tube 14 from a preselected angle of elevation is sensed by elevation sensor 54 which transmits, as schematically shown in FIG. 2, a related unbalanced fluidic signal to a fluidic amplifier 58 from a low pressure fluid supply 60. whereby, correspondingly unbalanced high pressure fluid from a high pressure supply 62 is'transmitted to opposite ends of cylinder 24 for related actuation of piston 26, and thereby piston rod 28 and tube 14, until elevation sensor 54 transmits a balanced fluid signal to fluidic ampli fier 58 when the pre-selected elevation angle of the tube is recovered.

Sight recovery system 20 also comprises an azimuth recovery system 64 which includes in cooperation with carrier 32 a piston 66 which is disposed in a cylinder 68 that extends laterally through base 40 of yoke 38, as shown in FIG. 9, for relative sliding displacement respective thereto. A cap 70 closes fluid tight each end of cylinder 68 and a shaft 72 extends through piston 66 with extending ends 73 thereof being slidingly received by cooperating bores 74 through the respective caps. A seal 76 in each of the bores 74 makes fluid tight contact with the respective ends 73 and each thereof is fixed to a flange 78 extending from carrier 32 to fix piston 66 thereto. Thus, when pressurized fluid introduced into the cylinder 68 on the right side of piston 66, through port 80, is greater than that introduced on the left side of the piston through port 82, the excessive pressure acts against the respective cap 70 to move yoke 38 and thereby tube 14 to the right according to the amount of pressure introduced. The converse is also true when the pressurized fluid introduced into cylinder 68 on the left side of piston 66 through port 82 is greater than that introduced on the right side through port 80.

It is important that prior to the firing of projectiles from tube 14 piston 66 should be centrally located in cylinder 68 thereby permitting automatic recovery of the pre-selected azimuth adjustment if base plate 16 should be displaced laterally in either direction. This is accomplished manually by a worm gear 84 which is rotatingly mounted between flanges 78 and which includes at both ends a cylindrical portion 86 which is journalled to the respective ones of the flanges. Axial displacement of worm gear 84 is prevented, as shown in FIG. 9, by the contact of annular shoulders 88 at opposite ends thereof with the inside surfaces of the flanges. A knurled knob 90 is fixed to the end of each of the cylindrical portions 86 to provide for manual rotation of worm gear 84. Rotation of worm gear 84 is converted to lateral displacement of yoke 38 by means of a plunger 92 which is slidingly disposed in the yoke so that end 94 thereof is displaceable in and out of heli cal channel 96 of the worm gear. Thus, when plunger 92 is pushed inwardly to engage end 94 with channel 96 the rotation of worm gear 84 moves yoke 38 laterally to where piston 66 is centrally located in cylinder 68. When plunger 92 is pulled outwardly so as to disengage end 94 thereof from channel 96, yoke 38 is free to be moved laterally by the differential fluid pressure introduced into cylinder 68 as hereinafter described.

The introduction of fluid pressure into cylinder 68 is controlled by a fluidic system 98, such as is shown schematically in FIG. 1 and which is more specifically disclosed in the copending patent application by Herbert J. Lewis and myself entitled Fluid System with Angular Displacement Sensor for Axially Reciprocating Shaft, Ser. No. 249,334; filed May l, 1972. Fluid system 98 comprises an azimuth sensor 100 which is mounted on the top end of cylinder 24 and is responsive to the angular displacement of piston rod 28. When tube 14 is aligned with the target at an adjusted azimuth setting a balanced signal is transmitted by sensor 100 from low pressure fluid supply 60 to a fluidic amplifier 102 whereby high pressure fluid from high pressure supply 62 is applied equally to opposite sides of piston 66 through ports 80 and 82 to hold the tube at the adjusted azimuth setting. When piston rod 28 is angularly displaced by a lateral component of movement, when a projectile is fired from tube 14 or for any other reason, a related unbalanced signal is transmitted by sensor 100 to fluidic amplifier 102 thereby varying proportionally the high pressure fluid transmitted to cylinder 68 through ports 80 and 82. Responsive to this unbalanced high fluid pressure, piston 66 is moved to reposition tube 14 at the adjusted azimuth setting where the signal transmitted from sensor 100 is balanced.

it is, of course, important that sensor 100 generate and transmit to fluidic amplifier 102 signals at all positions of piston rod 28 during axial travel relative to cylinder 24 so that azimuth recovery system 64 may be responsive to any lateral component of movement of tube 14 and correct the deviation from the adjusted azimuth setting simultaneously with the correction of the angle of elevation by elevation recovery system 52. This is possible with sensor 100 which, as shown in FIGS. 7 and 10, is fully described in the aforereferenced copending application by Lewis and myself, includes a cylindrical body 103 which is rotatingly mounted to the upper end of cylinder 24 and in which there is provided an axial bore 101 that receives, as by press fit, a sleeve 104. Sleeve 104 is provided with an axial bore 106 which receives piston rod 28 with a close sliding fit. The cylindrical circumference of piston rod 28 is interrupted by a chordal surface 108 thereby forming, along the length of the piston rod that passes through bore 106 during reciprocal displacement thereof, angular edges 110 and 112 at the junctions of the sides of the chordal surface with the cylindrical circumference of the piston rod. Such chordal surface 108 also defines, with the surface of bore 103, an exhaust chamber 114 which is open at both ends to exhaust fluid therefrom.

Located in sleeve 104 equidistant from the opposite ends thereof is a pair of segmental channels 115 which are angularly related to each other with ends 116 thereof spaced closer together than the opposite ends. Channels 115 are closed by the circumferential surface of bore 101 to form a right chamber 118 and a left chamber 120. A closed end well 122 extends radially through body 103 into sleeve 104 and such well has fluid communication with right chamber 118 and left chamber 120 by means of apertures 124. A right nozzle 126 provides communication between right chamber 118 and bore 106 and a left nozzle 128 provides communication between left chamber 120 and such bore. Nozzles 126 and 128 are so located that they are bisected by edges 110 and 112 when tube 14 is indexed at the adjusted azimuth setting, as shown in FIG. 10. When tube 14 is displaced from the adjusted azimuth setting, and piston rod 28 is consequently rotated relative to sleeve 104, one of the nozzles 126 or 128 is fully open to exhaust chamber 114 and the other is fully blocked by the cylindrical circumference of piston rod 28. Communication between well 122 and low pressure fluid supply 60 is made by conduit 130. An aperture 132 extends through body 103 for fluid communication with right chamber 118 and a similar aperture 134 extends through the body for fluid communication with left chamber 120 and such apertures communicate by means of conduits 136 and 138, respectively, with input legs 140 and 142 of fluidic amplifier 102.

Fluidic amplifier 102, as shown in FIG. 1, is of conventional type and comprises a junction 143 to which high pressure fluid is conducted by conduit 144 from high pressure fluid supply 62. Two branches, including a right passage 146 and a left passage 148, extend from a junction 143 in a Y configuration so that the high pressure fluid may be directed at such junction into either passage with practically no loss in momentum, in

a well-known manner. Proportional deviation of the high pressure fluid flow into passages 146 and 148 is controlled by the low pressure fluid conducted from apertures 132 and 134 to input legs 140 and 142, respectively, by the related conduits 136 and 138. Legs 140 and 142 of fluidic amplifier 102 are located in the same plane as passages 146 and 148 and extend into junction 150 from opposite sides thereof so as to direct the low pressure fluid from legs 140 and 142 against the high pressure fluid from high pressure supply 62 at the junction whereby, the high pressure fluid is divided between passages 146 and 148 according to the low pres sure fluid transmitted from sensor 100. The high pressure fluid is conducted from right passage 146 and left passage 148 to ports and 82 in cylinder 68 by conduits 154 and 156, respectively, to displace piston 66 in cylinder 68 according to the signals generated in sensor by the angular displacement of piston rod 26 with tube 14.

A bracket 157 is mounted to cylinder 24 for rotatable support of a worm gear 158 which is provided with knobs 160 for manual rotation thereof. Worm gear 158 has meshing engagement with a segmental rack having a plurality of teeth 162 around body 103 so that rotation of a knob 160 is transferred to such body. Thereby, azimuth sensor 100 is initially zeroed respective to an adjusted azimuth setting of tube 14, with edges and 112 equally bisecting nozzles 126 and 128 as shown in FIG. 10, and fluidic system 98 will automatically return tube 14 to the adjusted azimuth setting whenever displaced therefrom.

OPERATION Mortar 12, when set up for firing, is sighted roughly upon the selected target. The fine vernier adjustment, as to the azimuth setting, is made by turning either knob which rotates body 103, and thereby sleeve 104, relative to piston rod 28. Thus, the changing zero setting effected by the rotation of sleeve 104 varies ac cordingly the azimuth position of tube 14 by lateral displacement of yoke 38, through fluidic system 98, to where the tube is aligned with the target as shown in FIGS. 4 and 4a wherein line x-x represents the axis of tube 14 and line yy represents the predetermined azimuth adjustment. The fine adjustment of tube 14 to the pre-selected angle of elevation is similarly achieved through elevation recovery system 52 by means disclosed in the aforementioned patent application by LaSpisa et al, and not a part of this invention. When tube 14 is sighted on the selected target, either knob 90 is turned to center piston 66 in cylinder 68 as indicated by suitable markings (not shown) on carrier 32 and yoke 38.

Responsive to the the discharge of a projectile from tube 14, the recoil force generated thereby drives base plate 16 rearwardly, if not firmly embedded in the supporting terrain. Depending upon the character of the supporting terrain, the rearward displacement will comprise both a rearward componenet of movement and a lateral component. The rearward component will cause angular displacement of rotor 56 and immediate response of elevation recovery system 52 to recover the preselected angle of elevation. Simultaneously therewith, the lateral component of the rearward displacement, as shown in FIGs. 5 and 5a, displaces tube axis x-x angularly respective to the pre-determined azimuth adjustment y-y causing piston rod 28 to rotate and signal azimuth recovery system 64 for immediate realignment with the target, as shown in FIGS. and 5a.

Although the present invention is explained in accordance with the preferred embodiment shown and described herein, it will also become obvious to persons skilled in the art that other forms thereof as well as changes in the particular forms described, are possible within the spirit and scope of the present invention. Therefore, it is desired that the present invention shall not be limited except insofar as it is made necessary by the prior art and by the spirit of the appended claims.

I claim:

1. ln a cannon comprising a projectile firing tube, a base plate for supporting the rear end of said tube, and a tripod for supporting the front end of said tube in an elevated position, said base plate being subject to displacement in response to a projectile being fired from said tube, said displacement comprising a rearward component of movement and a lateral component of movement, the improvement which comprises a system for automatically realigning the cannon with a target when displaced therefrom, said system including an elevation recovery system for automatically returning the cannon to a preselected angle of elevation when displaced therefrom, said elevation recovery system being operationally responsive to the rearward component of movement, and an azimuth recovery system operationally responsive to the lateral component of movement and being operationally disposed in cooperation with said elevation recovery system for automatically, and simultaneously therewith, returning the cannon to a predetermined azimuth adjustment.

2. The invention as defined in claim 1 wherein said azimuth recovery system comprises mechanical means for sensing the lateral component of movement and fluid signalling and actuating means responsive to said mechanical sensing means for automatically returning said tube to the predetermined azimuth adjustment.

3. The invention as defined in claim 2 wherein said elevation recovery system includes means for varying the elevation of said tube, and said mechanical means of said azimuth recovery system is responsive to displacement of said means for varying the elevation of said tube for sensing the lateral component of movement.

4. The invention as defined in claim 3 wherein said elevation recovery system includes a fluid actuated piston rod, a yoke carried by said piston rod and a sleeve carried by said yoke and mounted on said tube for converting linear displacement of said piston rod to corresponding changes in the angle of elevation of said tube, and wherein said sensing means of said azimuth recovery system is responsive to angular displacement of said piston rod by the lateral component of movement of said base plate.

5. The invention as defined in claim 4 wherein said azimuth recovery system includes a carrier fixed to the extending end of said piston rod, and means slidingly mounting said yoke for lateral displacement relative to said piston rod.

6. The invention as defined in claim 5 wherein said yoke is disposed for lateral movement responsive to said actuating means.

7. The invention as defined in claim 6 wherein said actuating means includes a piston fixed to said carrier, a fluid receiving cylinder laterally disposed in said yoke for slidingly accommodating said piston, and means for applying pressurized fluid to said cylinder on opposite sides of said piston responsive to said signaling means.

8. The invention as defined in claim 7 and including means for manually moving said yoke laterally to center said piston in said cylinder.

9. The invention as defined in claim 8 wherein said means for centrally locating said piston in said cylinder comprises a worm gear disposed in said carrier for manual rotation, and means mounted to said yoke for releasable engagement with said worm gear for converting rotation thereof to lateral displacement of said yoke on said carrier. 

1. In a cannon comprising a projectile firing tube, a base plate for supporting the rear end of said tube, and a tripod for supporting the front end of said tube in an elevated position, said base plate being subject to displacement in response to a projectile being fired from said tube, said displacement comprising a rearward component of movement and a lateral component of movement, the improvement which comprises a system for automatically realigning the cannon with a target when displaced therefrom, said system including an elevation recovery system for automatically returning the cannon to a preselected angle of elevation when displaced therefrom, said elevation recovery system being operationally responsive to the rearward component of movement, and an azimuth recovery system operationally responsive to the lateral component of movement and being operationally disposed in cooperation with said elevation recovery system for automatically, and simultaneously therewith, returning the cannoN to a predetermined azimuth adjustment.
 2. The invention as defined in claim 1 wherein said azimuth recovery system comprises mechanical means for sensing the lateral component of movement and fluid signalling and actuating means responsive to said mechanical sensing means for automatically returning said tube to the predetermined azimuth adjustment.
 3. The invention as defined in claim 2 wherein said elevation recovery system includes means for varying the elevation of said tube, and said mechanical means of said azimuth recovery system is responsive to displacement of said means for varying the elevation of said tube for sensing the lateral component of movement.
 4. The invention as defined in claim 3 wherein said elevation recovery system includes a fluid actuated piston rod, a yoke carried by said piston rod and a sleeve carried by said yoke and mounted on said tube for converting linear displacement of said piston rod to corresponding changes in the angle of elevation of said tube, and wherein said sensing means of said azimuth recovery system is responsive to angular displacement of said piston rod by the lateral component of movement of said base plate.
 5. The invention as defined in claim 4 wherein said azimuth recovery system includes a carrier fixed to the extending end of said piston rod, and means slidingly mounting said yoke for lateral displacement relative to said piston rod.
 6. The invention as defined in claim 5 wherein said yoke is disposed for lateral movement responsive to said actuating means.
 7. The invention as defined in claim 6 wherein said actuating means includes a piston fixed to said carrier, a fluid receiving cylinder laterally disposed in said yoke for slidingly accommodating said piston, and means for applying pressurized fluid to said cylinder on opposite sides of said piston responsive to said signaling means.
 8. The invention as defined in claim 7 and including means for manually moving said yoke laterally to center said piston in said cylinder.
 9. The invention as defined in claim 8 wherein said means for centrally locating said piston in said cylinder comprises a worm gear disposed in said carrier for manual rotation, and means mounted to said yoke for releasable engagement with said worm gear for converting rotation thereof to lateral displacement of said yoke on said carrier. 